Radio frequency identification (RFID) systems generally consist of at least one host reader and a plurality of passive transponders, which are commonly termed credentials, cards, tags, or the like. An essential function of the host reader is to “excite” or power up the transponder. The reader transmits high voltage excitation signals into the surrounding space, which are received by a transponder proximal to the reader and which provide operational electrical power for the internal electronics of the recipient transponder. Once the transponder is powered up, it communicates information to the reader in a contactless manner. In particular, the powered up transponder transmits communication signals in the form of electromagnetic waves into the surrounding space which are received by the reader. Accordingly, all of the transponders and readers each have a transmitting function and a receiving function.
The transmitting and receiving functions may be performed by separately dedicated discrete electronic components, but more commonly at least some of the electronic components are combined into integrated circuits or shared between different functions within the transponder or reader to reduce manufacturing costs. For example, the internal electronics of the transponder may be limited to a single antenna for transmitting transponder signals and receiving excitation signals and to an integrated circuit for performing the remaining necessary operational functions of the transponder.
A low-cost reader may desirably include an application specific integrated circuit (ASIC), which is an integration of low voltage-rated components exclusively, since low voltage-rated components are generally significantly less expensive than high voltage-rated components. Nevertheless, a reader incorporating an ASIC is only cost-effective if the number of additional electronic components needed to complete the internal electronics of the reader are minimized. A low-cost reader may also desirably employ a single antenna to perform both the excitation signal transmitting function and the transponder signal receiving function. However, this results in the superposition of the smaller voltage transponder signal on the higher voltage excitation signal which requires standoff of the high voltage excitation signal from the low voltage-rated components included within the internal electronics of the reader.
Standoff of the high voltage excitation signal is generally the single most difficult function to incorporate into an ASIC of the reader because multiple high voltage-rated components, which are not permitted in an ASIC, are usually required for the standoff function. Accordingly standoff is typically performed by special circuitry upstream of the ASIC, which strips the transponder signal off the excitation signal before the transponder signal is amplified and detected by the reader. Many prior art techniques for stripping the transponder signal utilize special diode detection circuitry, which usually requires several high voltage-rated components (e.g., diodes, resistors and capacitors) to tolerate the relatively high voltage across the reader antenna. Such high voltage-rated components are relatively expensive and cannot be incorporated into an ASIC as noted above.
The present invention recognizes a need for alternate low-cost receiving circuitry in a reader which effectively strips a low voltage transponder signal off of a high voltage excitation signal. Accordingly, it is an object of the present invention to provide a low-cost, effective low voltage signal stripping circuit. More particularly, it is an object of the present invention to provide a low voltage signal stripping circuit which requires a minimal number of high voltage components. It is another object of the present invention to provide a low voltage signal stripping circuit having a plurality of low voltage components which are incorporated into an ASIC.
These objects and others are accomplished in accordance with the invention described hereafter.